In current wireless communications systems such as Long Term Evolution (LTE) and Long Term Evolution-Advanced (LTE-A) communications systems, base stations (for example, eNBs) generally use a Multiple-Input Multiple-Output (MIMO) technology based on multiple antennas. The MIMO technology may be used to concurrently transmit data of multiple layers in a same time-frequency resource in space or transmit a data stream at multiple layers in space, so that a compromise is achieved between multiplexing and diversity. Regardless of whether multiplexing or diversity is used, the multi-antenna MIMO technology provides additional resources and degrees of freedom in a space domain. Both theoretical research and engineering practice fully prove that the spectral efficiency of a wireless communications system can be greatly improved by using these resources and degrees of freedom.
When concurrent data of multiple streams is concurrently transmitted in space, a receiver needs to demodulate the concurrent data of multiple layers. Currently, in all mainstream wireless communications systems, coherent demodulation based on a reference signal is used, and a receiver needs to estimate downlink channel state information by using a specific reference signal, and feed back corresponding information to a base station. Therefore, a channel state information reference signal (CSI-RS) for estimating downlink channel state information is critical in a multi-antenna MIMO wireless communications system. In particular, when there are more antennas, higher requirements are imposed on reference signal design. In an LTE Release 8 (Rel-8) system, a configuration with a maximum of four antenna ports is supported, and a demodulation reference signal in use is a cell-specific reference signal (CRS) based on an antenna port. In LTE Release 10 (Rel-10), a maximum of eight antenna ports is introduced, and in addition, to support CSI measurement on the eight antenna ports, a CSI-RS reference signal is introduced. CSI-RS signals actually used may be configured according to requirements, but a maximum quantity of antenna ports cannot exceed eight that is currently defined in a protocol. In actual use, generally, a quantity of required CSI-RS antenna ports depends on a quantity of physical antennas.
However, in a currently researched two-dimensional antenna array, a quantity of antennas far exceeds 8 that is currently defined in a system. For example, a maximum quantity of antennas on a base station side may reach 12, 32, or 64. Theoretical research proves that if the quantity of antennas on the base station side is greater, a greater performance gain can be achieved in the system. To implement use of more antennas on the base station side, the system needs to extend a method for transmitting and using a CSI-RS reference signal. Currently, due to a maximum of eight CSI-RS antenna ports, more transmit antennas cannot be supported.